A mass grave containing 100 decapitated bodies has been discovered inside a school south of the Islamic State-held city of Mosul, according to the Iraqi military, which is conducting a US-backed operation to retake the jihadist stronghold.

Iraqi soldiers came across the grave in the School of Agriculture in the town of Hammam al-Alil that was recaptured from Islamic State (IS, formerly ISIS/ISIL) terrorists earlier on Monday, spokesman for Iraq’s Joint Military Command Brig. Gen. Yahya Rasool said in a statement cited by AP.

Most of the bodies have been reduced to skeletons. Due to the decaying process of the bodies as well as their clothes it’ is hard to establish whether these were soldiers or civilians, Rasool added. A forensics team from Baghdad is set to investigate the site and carry out tests on the bodies.

Islamic State terrorists and all those involved in the atrocities in Mosul should be hold responsible, UN spokesman Stephane Dujarric said following the news about the latest mass grave.

“The only thing we can do is condemn these atrocities. We hope that the military operation will be over soon and that it would go in accordance with international law, human rights and international humanitarian law so that people could get their lives back to normal. There is no doubt that those who commit these inhuman crimes should be held accountable,” Dujarric said, adding that the UN is highly concerned about the humanitarian situation in Mosul.

In June, Iraqi troops found another mass grave with the remains of nearly 400 people by the city of Fallujah while leading an operation against IS. Most of the victims there were executed in 2014 and 2015 with a shot in the head.

Mosul, the IS stronghold in Iraq, is under siege by several forces, including the Iraqi army, its allies from the Shiite militias, Kurdish militias, and a Turkish expeditionary force in Iraq. The US-led coalition is providing air support for the joint offensive.

The current events resemble a “medieval slaughter” as the population is being killed by both IS terrorists and the US-led coalition conducting operations in Mosul, according to spokeswoman for Russia’s Foreign Ministry Maria Zakharova.

The UN Office for the Coordination of Humanitarian Affairs said on Sunday that 34,000 Iraqis had left their homes since the start of the operation, adding that most of those people were put in temporary shelters. However, over 1 million civilians are believed to remain trapped in Mosul as the terrorists kill anybody who attempts to flee.

While the offensive in Iraq’s second most populous has not yet entered the intensive urban warfare phase, international humanitarian organizations have voiced concern that the hostilities will take a significant toll on the civilian population.